The present invention relates to an improved gaseous pollutant abatement reactor system, such as a regenerative thermal oxidizer or selective catalytic reduction system including a gas reaction chamber, wherein the structured media bed in the reaction chamber comprises a plurality of ceramic blocks each having aligned gas passages extending through the end faces of the blocks and wherein the blocks include a chamfered surface extending at an angle relative to the side faces of the blocks which promotes gas flow between the blocks at spaced locations and equalizes the pressure within the reaction chamber.
Regenerative thermal oxidizers or RTOs are now widely used for oxidizing gaseous pollutants including volatile organic compounds, such as hydrocarbons, in waste or exhaust gas streams. A typical regenerative thermal oxidizer includes at least two heat exchange chambers each having a heat exchange media bed therein and a combustion chamber communicating with the heat exchange chambers. The waste gas stream is directed alternatively or periodically into one of the heat exchange chambers, which has been previously heated, and wherein the gaseous pollutant is oxidized. The gas then flows into the combustion chamber, wherein any remaining pollutants are oxidized. The combustion chamber is also used to preheat the gas flowing through the regenerative thermal oxidizer during startup and oxidize any remaining pollutants in the waste gas stream. The cleansed heated gas then flows into the second heat exchange chamber, heating the media in the second heat exchange chamber and the cleansed gas is vented to atmosphere. The gas flow through the regenerative thermal oxidizer is then reversed, such that the waste gas flows into the heat exchange media in the second heat exchange chamber, oxidizing the pollutants, etc. A regenerative thermal oxidizer thereby conserves heat resulting in a more efficient system.
There are basically two types of regenerative thermal oxidizers. The first type includes a plurality of generally spaced heat exchange chambers and a combustion chamber, generally located above the heat exchange chambers, and communicating with the chambers. A series of control valves then directs the gas through the heat exchange chambers as described above. The regenerative thermal oxidizer of this type may include three heat exchange chambers, wherein the exhaust gas is directed into one of the chambers for oxidation and the heated cleansed gas is directed into a second chamber, heating the heat exchange media in the second chamber as described above. The third chamber serves as a purge chamber, wherein any remaining pollutants in the heat exchange media are removed and generally circulated to the inlet of the regenerative thermal oxidizer or the purge gas is directed into the combustion chamber. By alternating the flow through the regenerative thermal oxidizer through the three heat exchange chambers, the pollutants in the process or exhaust gas is removed and oxidized without exhausting pollutants to the atmosphere and the heat exchange media is periodically cleaned.
A second type of regenerative thermal oxidizer, sometimes referred to as a rotary valve regenerative thermal oxidizer, includes a plurality of pie-shaped heat exchange chambers enclosed generally by a cylindrical wall or housing. A rotary valve is located below the heat exchange chambers which rotates to direct the gas containing the pollutants into a first adjacent series of heat exchange chambers, wherein the gas is heated and the pollutants are oxidized as described. A combustion chamber is located above the heat exchange chambers which receives the gas and removes any remaining pollutants as described above. The gas then flows downwardly through a second series of adjacent heat exchange chambers, wherein the heat exchange media bed is heated and the cleansed gas is vented to atmosphere. The remaining heat exchange chambers, usually located between the inlet heat exchange chambers and the outlet heat exchange chambers, serve as purge chambers, wherein the heat exchange media is cleaned. A rotary valve regenerative thermal oxidizer may utilize an inlet purge, wherein clean air is directed into the purge chambers, removing pollutants upwardly into the combustion chamber. Alternatively, some of the heated cleansed gas from the combustion chamber may be directed downwardly through the purge chambers, removing the pollutants in the purge chambers, and the pollutants are then generally circulated back to the inlet of the regenerative thermal oxidizer. The valve is continuously rotated to continuously change the function of the heat exchange chambers, providing continuous operation in a single vessel.
The media may also include or be coated with a catalyst resulting in a catalytic reaction within the reaction chamber to remove certain pollutants. As used herein, the term regenerative thermal oxidizer is intended to include conventional regenerative thermal oxidizers having ceramic media and ceramic media coated with a catalyst, sometimes referred to as catalytic reactors.
Another type of gaseous pollution abatement system is commonly referred to as a selective catalyst reduction or SCR system used primarily to treat NOx, including NO and NO.sub.2. The ceramic media may be coated with a suitable catalyst or the catalyst may be mixed with the ceramic matrix prior to firing. Typical catalysts include noble metal catalysts, such as platinum, and base metal catalysts, such as vanadium or manganese oxide or Zeolite. A typical selective catalytic reduction system includes only one reaction chamber filled with a catalytic media bed as described. The gas to be treated flows through the bed of catalytic media in the reaction chamber where the NOx is reduced to nitrogen gas and nonpolluting oxides. Ammonia gas or other reducing agent may be introduced into the gas to effect NOx reduction. As used herein, the term gaseous pollution abatement system is intended to cover both regenerative thermal oxidizers and selective catalytic reduction systems.
Typically, gaseous pollution abatement systems have utilized a plurality of small elements of ceramic material as heat exchange or reaction media. The ceramic elements have included one inch ceramic saddle-shaped pieces or irregular mineral spheroids or gravel. Typically, the saddles or spheroids are poured into the heat exchange chambers and raked to form a bed of uniform depth. The individual pieces of the heat exchange media remain in whatever orientation they happen to fall into when the chamber is filled. The resistance to flow or pressure drop through the bed is relatively high and will vary through the media, depending upon the random orientation of the media and, to some extent, the degree of contamination. In a typical RTO or SCR having randomly oriented saddle-shaped ceramic elements, the overall pressure drop will be about ten inches of water, or greater.
More recently, loose particulate ceramic media has been replaced by structured ceramic media, such as disclosed in U.S. Pat. Nos. 5,707,229, 5,352,115 and 5,393,000 assigned to the assignee of the present application. As disclosed in these patents, the media bed may comprise blocks of ceramic material having generally parallel passages. The blocks of ceramic media, sometimes referred to as prismatic or prism-shaped blocks, have flat parallel rectangular end faces, perpendicular rectangular side faces and the parallel gas passages extending through the blocks through the end faces. The ceramic blocks are then stacked in side-to-side and end-to-end relation within the heat exchangers with the gas passages through the end faces generally coaxially aligned forming a bed of ceramic prismatic heat exchanger blocks. The gas to be treated flows upwardly through the parallel gas passages in the "inlet" heat exchange chamber or chambers, where the gas is heated and the pollutants oxidized as described above. The gas then flows upwardly into the combustion chamber and the heated cleansed gas flows downwardly from the combustion chamber through the ceramic monolith of prismatic ceramic blocks in the "outlet" heat exchange chamber or chambers which heats the ceramic media as described.
The gas passages through the parallel passages in the blocks of media bed typically have a diameter of less than one inch or more preferably between 0.1 to 0.25 inches. The dimension of the gas passages are selected to achieve the desired cross-sections for a particular application. The gas passages generally comprise at least 40 percent of the cross sectional area of the blocks and the pressure drop across the ceramic media bed is more than three inches of water. More preferably, the gas passages account for 50 to 80 percent of the total cross-sectional area and most preferably about 70 to 80 percent of the cross-sectional area.
A bed of structured heat exchange media comprised of prismatic ceramic heat exchange blocks as described above has several advantages over loose particulate ceramic elements as set forth in the above-referenced U.S. Patents. First, a greater degree of thermal or reaction efficiency is achieved with a lower pressure drop across the bed. Further, the pressure drop using prismatic structured media is predictable and does not depend upon the random orientation of the particulate ceramic media providing a more uniform flow through the media bed. Further, contaminates are less likely to be entrapped in the small parallel passages than within the interstices between the small, irregularly-shaped ceramic pieces. Finally, it has been found that purging of contaminates in the bed of structured media during the purge cycle is more efficient and sometimes more complete. This permits the system to operate with increased inlet and outlet times with a reduced amount of purge gas in a reduced purge cycle time. The use of prism-shaped structured media of this type, either coated with a suitable catalyst or having a catalyst mixed within the ceramic matrix, has similar advantages in a selective catalytic reduction system.
However, prism-shaped structured media of this type also has several important disadvantages. As set forth above, the prism-shaped ceramic blocks, which typically have a height of 300 mm and end faces having a width and length of the 150 mm, are stacked together to construct a bed of the desired cross-sectional area and height, wherein the gas passages through the end faces are aligned as described above. The disadvantages relate to the confined gas flow through the aligned gas passages and the constrained laminar flow through the height of the bed. The aligned passages through the stacked prismatic blocks prevents any cross-flow or redistribution of the gas within the bed. Consequently, any flow maldistribution at the entrance remains uncorrected and reduces the overall effectiveness or efficiency of the structured media because the ratio of the flow rate to the volume of structured media is not optimum. As will be understood by those skilled in this art, the pressure across the inlet to the structured media is generally not uniform. Thus, for example, a greater percentage of the contaminated gas may flow through the stacked bed of prismatic structured media adjacent the inlet resulting in maldistribution at the entrance. This maldistribution of gas results in reduced thermal efficiency and a greater percentage of contaminates may collect in the stacks of prismatic ceramic blocks adjacent the inlet. To correct this maldistribution of gas, a longer purge cycle may be required in regenerative thermal oxidizers. Further, the small dimensions of the gas passages through the prism-shaped ceramic blocks limits the Reynold's number of the gas flow through the passages to the laminar regime and thus limits the heat transfer coefficient.
The use of prism-shaped structured catalytic media in a selective catalytic reduction system also suffers a further disadvantage. Because the destruction efficiency of the bed is dependant upon uniform distribution of the gas across the bed, the maldistribution of gas through a bed of prism-shaped ceramic blocks also results in reduced destruction efficiency. That is, the use of structured catalytic media results in both a reduction in thermal and destruction efficiency.
Thus, as will now be understood, although the prism-shaped ceramic blocks now used in structured media beds provides several advantages over loose particulate ceramic media, the prismatic structured media is responsible for the confined flow problem described above because of the prism geometry, which requires that the gas passages be continuous through the packing. The present invention permits the use of nonprismatic structured media blocks, but eliminates the problems set forth above by permitting the gas to circulate between the blocks of nonprismatic ceramic media and equalize the pressure and gas flow within the reaction chamber. The gas flow is thus increased into the turbulent regime at spaced locations, improving the heat transfer coefficient and substantially eliminating the gas flow maldistribution described. Providing more uniform gas flow through the bed is also particularly advantageous in selective catalytic reduction systems, improving the destruction efficiency of the system.